The present invention relates to a method for the continuous preparation of explosive mixtures in dual screw mixers. The method makes it possible to mix the proportionally fed solid and liquid components uniformly with one another at varying temperatures and varying mixing and kneading intensities in successive transport and mixing zones within the mixer.
Up to the present time it has been the most common practice in the explosives industry to work components in apparatus operating batch-wise to form a very homogeneous mass. These apparatus are relatively large units having capacities of about 200 to 700 kilograms.
The mixing and kneading performed in the actual mixing and kneading apparatus is accomplished by means of mechanical devices operating on the mixing paddle or helical blade principle. Aside from the disastrous effects produced in the case of accidents that can be attributed to the great size of the batches, these mixing and kneading apparatus have one decided disadvantage. The design principles mentioned above have as a consequence that the mechanical devices always have a geometry designed for a particular purpose, and that geometry can not be changed. This means that different mixing and kneading apparatus have to be used for different explosives.
Furthermore, there are difficulties involved in producing a mixture of high homogeneity in batch apparatus. Therefore there exists in practice the danger of the formation of pockets of unmixed components.
It is for this reason that manufacturing methods or apparatus have been described for the continuous preparation of explosive mixtures using screw mixers (cf. U.S. Pat. No. 3,997,147, DE-OS No. 2,510,022 and DE-OS No. 2,515,492).
These known methods have common points to the extent that they perform the actual mixing and kneading process in a dual screw mixer which occasionally operates on the helical paddle mixer principle. Helical paddle mixers in a dual screw arrangement consist either of a continuous helical blade or of paddles in a helical array.
These types of helical mixers have decided disadvantages. The residence time range (residence time performance) of each model of machine is very narrow, and can be varied only by changing the rotatory speed. The latter, however, for reasons of safety, must not be too high. It is not possible, therefore, to change the detention time by any simple manipulation. The insertion of so-called baffles, or the use of progressively cut helices improves the mixing effect only slightly. On the relatively short course through the machine it is difficult to produce a mixture of great homogeneity, especially if gelatination and crosslinking is desired for a particular explosive mixture.
The components which are to be worked therefore undergo always a more or less constant stress on account of the virtually constant shear gradient resulting in poor variability of the shear forces. If these shear forces are additionally great for the purpose of achieving a sufficient mixing action in a relatively short machine length, then the hazard is increased to an undesirable extent, and gel structures already formed in the mixture can be torn apart again.
The previously mentioned narrow residence time range in conventional screw mixers has furthermore the disadvantage that fluctuations in the feeding of individual components can be compensated only to a slight extent, so that inhomogeneities can develop.
The problem therefore existed, in the continuous production of explosive mixtures in screw mixers, of avoiding the above-described disadvantages and being able to control the mixing process such that it can be used for explosive mixtures of differing composition, while obtaining mixtures of high homogeneity. Furthermore, the process must be able to be so conducted as to minimize the hazards.